Triaxial test device and method based on salt dissolution erosion

Abstract

The invention discloses a triaxial test device and method based on salt dissolution erosion. The test device comprises a pressure cavity, an axial loading system, a confining pressure control system, an erosion rate control system capable of continuously injecting constant-temperature water into a sample according to a preset rate, a salt solution collection system for judging whether salt particles in the sample are dissolved or not, and a monitoring system, a saturated salt water tank for injecting saturated salt water into the sample and enabling the sample to reach a saturated state is arranged in the confining pressure control system. The test method comprises: preparing a sample according to requirements; vacuumizing the sample and injecting saturated salt water; applying confining pressure and axial pressure to complete consolidation of the sample; introducing water at a preset temperature to control the erosion rate; detecting the salt solubility of the discharged erosion liquid to judge whether the erosion liquid is completely corroded or not; and performing a drainage shear test on the sample. Saturated salt water is adopted to replace clean water for saturation and consolidation, interference of particle component change in the consolidation stage on test results is avoided, and stress-strain response caused by particle loss can be truly reflected.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of geotechnical engineering indoor unit test equipment, and particularly relates to a triaxial test device and method based on salt dissolution erosion. BACKGROUND

[0002] With the continuous advancement of water conservancy and hydropower development process in China, bedrock dam sites with excellent topographic and geological conditions have become increasingly scarce. In many water energy resource development bases located in large river basins, the technical problem of building dams on deep sand and gravel cover layers is generally faced. Due to the restriction of technical and economic conditions, the dam foundation cutoff wall is often difficult to reach the bedrock, and a suspended structure is mostly used, which leads to the fact that the dam foundation cannot be completely closed. Under the long-term seepage action, the fine particles in the sand and gravel cover layer are easy to migrate and lose, which is called erosion. The loss of fine particles not only increases the pore space of the soil and improves the permeability coefficient, but also reduces the effective contact area of the soil skeleton, thereby weakening the stability of the soil, so the research on the mechanical characteristics of sand and gravel erosion is very important.

[0003] The existing conventional sand and gravel erosion mechanical test mainly studies by adjusting the content of fine particles, and the specific methods mainly include two kinds: one is by means of hydraulic erosion, and the other is by artificially adjusting the gradation. However, the hydraulic erosion method is difficult to ensure the uniformity of the sample, and the artificial gradation adjustment method cannot reflect the true state of the initial pore of the soil. Some scholars also propose a method of replacing fine particles with salt particles, and then dissolving the salt particles in clean water to achieve the removal of fine particles. However, during the saturation and consolidation process, the salt particles may have been dissolved, which leads to the fact that the stress change in the whole erosion process of the soil cannot be accurately reflected. SUMMARY

[0004] The purpose of the present application is to provide a triaxial test device based on salt dissolution erosion and controllable erosion conditions. Another purpose of the present application is to provide a test method that can accurately reflect the stress change in the erosion process based on the triaxial test device.

[0005] Technical scheme: The triaxial test device based on salt dissolution erosion provided by the present application comprises an axial loading system having a pressure cavity and capable of applying an axial load to a sample, a confining pressure control system for applying a confining pressure to the sample, an erosion rate control system capable of continuously injecting constant temperature water into the sample at a preset rate, a salt solution collection system for collecting and judging whether the salt particles inside the sample are completely dissolved, and a monitoring system for receiving and processing various sensor signals in the axial loading system, the confining pressure control system, the erosion rate control system and the salt solution collection system. A saturated salt water tank is arranged in the confining pressure control system to inject saturated salt water into the sample and make the sample reach a saturated state.

[0006] Furthermore, the axial loading system also includes a servo loading frame, a displacement actuator, a force sensor for real-time acquisition of load data during axial loading, and a data acquisition unit. The pressure chamber includes a sample cylinder for containing the sample, a water inlet channel for supplying constant-temperature water to the saturated brine and erosion rate control system, an erosion liquid discharge channel connected to the brine collection system, a confining pressure control water pipe connected to the confining pressure control system and used to apply confining pressure to the sample, a pressure chamber drain pipe located at the bottom of the pressure chamber for discharging the medium inside the pressure chamber, an upper water outlet pipe located at the top of the pressure chamber, and permeable stones located at the upper and lower ends of the sample cylinder.

[0007] Preferably, the confining pressure control system further includes a distilled water storage tank and a pressurizing unit, wherein the pressurizing unit controls the distilled water storage tank to inject water into the pressure chamber to achieve the confining pressure conditions.

[0008] Furthermore, the erosion rate control system includes a water storage tank, a constant temperature chamber connected to the water storage tank and capable of controlling the water temperature, and a flow control component, which can control the amount of water injected into the sample and the injection rate.

[0009] Furthermore, the salt solution collection system includes a storage tank for collecting and temporarily storing the corrosive solution, and a salt analyzer installed inside the storage tank for detecting the salt concentration in the corrosive solution. The salt analyzer sends the detection data to the monitoring system. An overflow pipe is provided at the upper end of the storage tank. When the liquid level in the storage tank is higher than the overflow pipe, the corrosive solution is guided through the overflow pipe to an auxiliary storage bottle.

[0010] The present invention discloses a test method based on salt dissolution erosion, applied in a triaxial testing apparatus, comprising the following steps:

[0011] S1: The sand and gravel material to be tested is graded and sieved according to the predetermined gradation curve to obtain different gradation components that meet the particle size requirements. Solid salt particles are mixed with the sand and gravel material in a uniform manner according to the set dosage to form a mixture. The mixture is then loaded into the sample tube in six layers using a layered compaction method to ensure that the sample density meets the test requirements.

[0012] S2: Place the sample tube on the permeable stone, reduce the sample air pressure to a vacuum state through the pressurization unit, and inject saturated salt water into the sample to make the sample fully saturated.

[0013] S3: The confining pressure control system and the axial loading system apply graded confining pressure and axial pressure to the specimen respectively, and maintain the loading conditions until the specimen is consolidated; when the axial and volumetric deformation rates of the specimen decrease to within the threshold range, the consolidation is considered complete.

[0014] S4: The erosion rate control system introduces distilled water at a set temperature and flow rate into the bottom of the sample to dissolve the salt particles inside the sample and form an erosion solution.

[0015] S5: The corrosive liquid is discharged into the storage tank. The salt analyzer monitors the salt content of the discharged corrosive liquid in real time and sends the data to the monitoring system. When the salt content is less than 0.1%, it is determined that the salt particles inside the sample have been completely dissolved. The monitoring system continuously records the axial strain, volumetric strain and pore pressure changes of the sample.

[0016] S6: The specimen is subjected to drained shearing. The axial loading system applies axial force to the specimen at a constant displacement rate until the specimen reaches the critical state. The monitoring system records the specimen volumetric strain, pore pressure change and stress path during the entire shearing process.

[0017] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages:

[0018] (1) Saturated salt water is used for saturation and consolidation during the sample consolidation process. Since the salt particles will not dissolve in saturated salt water, it can ensure that the sample maintains the preset initial particle composition before consolidation and can accurately simulate the target stress state. Compared with the existing method of using water saturation which causes fine particles to be lost in advance and the stress conditions to be distorted, it effectively avoids the interference of particle composition changes during the consolidation stage on the test results.

[0019] (2) After consolidation, the saturated brine is replaced with clean water so that the salt particles are gradually dissolved while keeping the stress conditions unchanged. This can truly reflect the stress-strain response caused by particle loss. It avoids the local stress concentration and sample disturbance problems that are easily caused by traditional water flow erosion methods, and makes the test results closer to the particle erosion behavior under actual engineering conditions, and obtains the whole process of erosion-deformation evolution with high precision.

[0020] (3) The dissolution rate of salt particles inside the sample is precisely adjusted by using a constant temperature controlled stable water flow, and the salt content in the discharged solution is monitored in real time and continuously by using a salt analyzer, so as to realize the quantitative characterization and dynamic evaluation of the amount of salt particles dissolution and its dissolution rate. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the triaxial testing device of the present invention;

[0022] Figure 2 This is a schematic diagram of the pressure chamber structure in this invention;

[0023] Figure 3 This is a schematic diagram of the confining pressure control system structure in this invention;

[0024] Figure 4 This is a schematic diagram of the erosion rate control system structure in this invention;

[0025] Figure 5 This is a schematic diagram of the salt solution collection system in this invention. Detailed Implementation

[0026] The technical solution of the present invention will be further described below with reference to the accompanying drawings.

[0027] like Figure 1 , Figure 2 As shown, the present invention discloses a triaxial test device based on salt dissolution erosion, which consists of an axial loading system 2, a confining pressure control system 3, an erosion rate control system 4, a salt solution collection system 5, and a monitoring system 6.

[0028] The axial loading system 2 includes a pressure chamber 1, a servo loading frame, a displacement actuator, a force sensor 21, and a data acquisition unit 22. The servo loading frame and displacement actuator are existing technologies and are not shown in detail in this embodiment. One possible implementation is described here: the servo loading frame provides stable support for the entire axial loading system, bearing the axial reaction force generated during loading, preventing offset or vibration during load transfer, and ensuring the stability and accuracy of loading. The servo loading frame can adopt a high-rigidity portal frame structure, made of 45# steel or alloy structural steel. The frame columns and beams are fixed with bolts. An installation interface for the displacement actuator is reserved at the top of the frame, and the bottom is fixed to the base of the triaxial testing device with positioning pins to ensure that the loading axis is coaxial with the sample center axis. The displacement actuator consists of a servo motor, a ball screw, a guide rail, and a loading rod 23. The lower end of the loading rod 23 is connected to a permeable stone. The displacement actuator receives control commands from the monitoring system 6 and drives the ball screw to rotate via the servo motor, converting the rotational motion into the linear reciprocating motion of the loading rod, achieving constant displacement rate loading. Force sensor 21 collects load data in real time during the axial loading process, converts the mechanical signal into an electrical signal, and transmits it to monitoring system 6. Data acquisition unit 22 records data such as confining pressure, axial force, axial displacement, pore water pressure, and volume deformation in real time, and is connected to monitoring system 6.

[0029] like Figure 2As shown, pressure chamber 1 is a sealed cylindrical cavity made of high-strength transparent glass, which houses the sample cylinder and pressure medium. Permeable stones 17 are located at both ends of the sample cylinder 11, clamping the sample and maintaining its cylindrical shape during the test. The permeable stones are circular plates with a diameter matching that of the sample cylinder. The upper permeable stones evenly apply axial force to the sample, preventing localized stress concentration. The permeable stones are also connected to a pore water pressure sensor via a water pipe to monitor changes in pore water pressure at the top of the sample throughout the saturation, erosion, and shearing processes. The pressure chamber is equipped with multiple pipes, including a water inlet channel 12, an erosion liquid discharge channel 13 connected to the salt solution collection system 5, a confining pressure control water pipe 14 connected to the confining pressure control system 3 and used to apply confining pressure to the sample, a pressure chamber drain pipe 15 located at the bottom of the pressure chamber to discharge the medium inside the pressure chamber, and an upper water outlet pipe 16 located at the top of the pressure chamber. The slight discharge from the upper water outlet pipe 16 indicates that the medium inside the pressure chamber is full, thus completely emptying the pore air inside the sample. In the sample saturation step, saturated brine enters the sample through the water inlet channel 12, and the saturated brine gradually rises axially and completely fills the pores, so that the sample is completely saturated with saturated brine. After saturation, the confining pressure control system 3 injects water into the sample from the confining pressure control water pipe 14 to apply graded confining pressure to the sample. During the erosion process, constant temperature water enters the sample through the water inlet channel 12, reacts with the solid salt particles inside the sample to form an erosion liquid, and is discharged from the erosion liquid discharge channel 13 at the top of the pressure chamber. The pressure chamber drain pipe 15 is used to empty the medium in the pressure chamber after each step.

[0030] like Figure 1 , Figure 3 The diagram shows the confining pressure control system 3 of this invention, which consists of a saturated saline tank 31, a distilled water storage tank 32, and a pressurizing unit 33. The saturated saline tank 31 stores a saturated salt solution and is connected to a permeable stone base at the bottom of the sample via a water inlet channel 12. It can inject a salt solution of a preset concentration into the sample and, under negative pressure, fully saturate the pores within the sample. The distilled water storage tank 32 is connected to a pressure chamber and continuously injects distilled water into the pressure chamber under the drive of a pressure-stabilizing pump to apply a constant confining pressure to the sample. The pressurizing unit 33 is equipped with a display, a first pressure gauge 34, a second pressure gauge 35, a third pressure gauge 36, and a timing knob 37.

[0031] like Figure 4As shown, the erosion rate control system 4 includes a water storage tank 41, a constant temperature chamber 42 connected to the water storage tank 41 and capable of controlling the water temperature, and a flow control component 43. The constant temperature chamber 42 consists of an integrated heating / cooling liquid storage chamber, which can stably maintain the water temperature within the range of 5~80℃. It has an internal water tank 44 for storing constant temperature water. The outer surface of the constant temperature chamber is equipped with a temperature adjustment knob 45, a pressure adjustment knob 46, and a timer adjustment knob 47. The bottom of the constant temperature chamber is equipped with an inlet pipe 48 connected to the water storage tank 41 and an outlet pipe 49 connected to the flow control component 43. The flow control component 43 uses a constant flow micro-pump, which can continuously inject constant temperature water into the sample at a preset rate and can also control the injection rate into the sample, effectively controlling the erosion conditions and achieving precise adjustment of the dissolution rate of salt particles inside the sample.

[0032] like Figure 5 As shown, the salt solution collection system 5 includes a storage tank 51 for collecting and temporarily storing the corrosive liquid, a salt analyzer 52, an overflow pipe 53, an upper vent 54 at the top of the storage tank, and a lower drain hole 55 at the bottom of the storage tank. After the test, the corrosive liquid in the storage tank is completely discharged through the lower drain hole 55. The storage tank 51 is made of corrosion-resistant transparent material and has internal volume markings for collecting and temporarily storing the salt solution seeping from the sample. The salt analyzer 52 is used to detect the salt concentration in the corrosive liquid. A conductivity-type salt analyzer can be used to detect the salt concentration of the corrosive liquid in real time and is connected to the monitoring system 6 via a Bluetooth module. When the liquid level in the storage tank is higher than the overflow pipe 53, the corrosive liquid is guided to an auxiliary storage bottle through the overflow pipe.

[0033] The monitoring system 6 is a computer terminal for data acquisition and centralized processing of the test device. It includes a data access module, a real-time processing module and a storage management module. The data access module is electrically connected to the sensors in the force sensor 21, pore water pressure sensor, flow meter 43, salt analyzer 52 and other devices to receive test data synchronously.

[0034] This invention also discloses a test method based on salt dissolution erosion, as follows:

[0035] The particle size distribution of the gravel material used in this experiment was determined by standard sieve analysis. The cumulative sieve residue percentages for each particle size are as follows: 100% for 20mm, 88% for 10mm, 60% for 5mm, and 38% for 2mm. For particles smaller than 2mm, the cumulative sieve residue percentage was determined by the measured values ​​from particle size analysis (particle size distribution test).

[0036] S1: The sand and gravel material to be tested is graded and sieved according to the predetermined gradation curve to obtain different gradation components that meet the particle size requirements; the sand and gravel within a certain particle size range is replaced with solid salt particles according to the set dosage, and dry stirring is carried out in the mixing tank to make the solid salt as uniform as possible in spatial distribution; the mixture is filled into the sample tube in six layers, and each layer is compacted with equal energy using a standard compaction hammer or vertical compaction device to make the sample density meet the test parameter requirements;

[0037] S2: Place the sample tube 11 on the permeable stone, and reduce the sample pressure to an approximate vacuum state (−70kPa~−90kPa) through the pressurization unit 33. Under vacuum conditions, inject saturated salt water into the sample to make the sample fully saturated.

[0038] S3: The confining pressure control system 3 and the axial loading system 2 apply graded confining pressure and axial pressure to the specimen respectively, and maintain the loading conditions until the specimen is consolidated; under each level of pressure, when the axial and volumetric deformation rates of the specimen decrease to within the threshold range (below 0.02% / h), the consolidation is considered to be complete.

[0039] S4: After consolidation and stabilization, distilled water at a set temperature (20℃, 40℃, 60℃) is introduced into the bottom of the sample through the erosion rate control system 4, which reacts with the solid salt particles inside the sample to form an erosion liquid.

[0040] S5: The corrosive liquid is discharged into the storage tank 51. The salt analyzer 52 monitors the salt content of the discharged corrosive liquid in real time and sends the data to the monitoring system 6. When the salt content is less than 0.1%, it is determined that the salt particles inside the sample have been completely dissolved. The monitoring system 6 continuously records the axial strain, volumetric strain and pore pressure changes of the sample.

[0041] S6: The specimen is subjected to drainage shearing. The axial loading system 2 applies axial force to the specimen at a constant displacement rate until the specimen reaches the critical state. The monitoring system 6 records the specimen volumetric strain, pore pressure change and stress path during the entire shearing process.

[0042] The parameters in the test steps can be changed according to the actual experimental needs. The test method of this invention uses saturated salt water instead of clean water for saturation and consolidation, which fundamentally avoids the interference of particle composition changes during the consolidation stage on the test results. The test device can also control erosion conditions and accurately obtain the stress changes of the soil erosion process under different conditions. The test results are close to the actual working conditions and accurate.

Claims

1. A triaxial testing apparatus based on salt dissolution erosion, comprising an axial loading system (2) having a pressure chamber (1) and capable of applying an axial load to the specimen, and a confining pressure control system (3) applying a confining pressure to the specimen, characterized in that, It also includes an erosion rate control system (4) that can continuously inject constant temperature water into the sample at a preset rate, a salt solution collection system (5) that collects and judges whether the salt particles inside the sample are completely dissolved, and a monitoring system (6). The monitoring system (6) receives and processes various sensor signals from the axial loading system (2), the confining pressure control system (3), the erosion rate control system (4), and the salt solution collection system (5). The confining pressure control system (3) is equipped with a saturated salt water tank (31) that injects saturated salt water into the sample and makes the sample saturated.

2. The triaxial testing apparatus according to claim 1, characterized in that, The pressure chamber (1) includes a sample tube (11) for containing the sample, a water inlet channel (12) for supplying constant temperature water to the saturated brine and erosion rate control system (4), an erosion liquid discharge channel (13) connected to the salt solution collection system (5), a confining pressure control water pipe (14) connected to the confining pressure control system (3) and used to apply confining pressure to the sample, a pressure chamber drain pipe (15) located at the bottom of the pressure chamber and discharging the medium in the pressure chamber (1), an upper water outlet pipe (16) located at the top of the pressure chamber (1), and permeable stones (17) located at the upper and lower ends of the sample tube respectively.

3. The triaxial testing apparatus according to claim 1, characterized in that, The axial loading system (2) also includes a servo loading frame, a displacement actuator, a force sensor (21) for real-time acquisition of load data during axial loading, and a data acquisition unit (22).

4. The triaxial testing apparatus according to claim 1, characterized in that, The confining pressure control system (3) also includes a distilled water storage tank (32) and a pressurizing unit (33), wherein the pressurizing unit (33) controls the distilled water storage tank (32) to inject water into the pressure chamber (1) to achieve the confining pressure condition.

5. The triaxial testing apparatus according to claim 1, characterized in that, The erosion rate control system (4) includes a water storage tank (41), a constant temperature chamber (42) connected to the water storage tank (41) and capable of controlling the water temperature, and a flow control component (43). The flow control component (43) is capable of controlling the amount of water injected into the sample and the injection rate.

6. The triaxial testing apparatus according to claim 1, characterized in that, The salt solution collection system (5) includes a storage tank (51) for collecting and temporarily storing the corrosive liquid, and a salt meter (52) installed in the storage tank (51) for detecting the salt concentration in the corrosive liquid. The salt meter (52) sends the detection data to the monitoring system (6).

7. The triaxial testing apparatus according to claim 6, characterized in that, The upper end of the storage tank (51) is provided with an overflow pipe (53). When the liquid level in the storage tank is higher than the overflow pipe, the corrosive liquid is guided to the auxiliary storage bottle through the overflow pipe.

8. A test method based on salt dissolution erosion, applied in the triaxial testing apparatus as described in any one of claims 1-7, characterized in that, Includes the following steps: S1: The sand and gravel material to be tested is graded and sieved according to the predetermined gradation curve to obtain different gradation components that meet the particle size requirements. Solid salt particles are mixed with sand and gravel material in a uniform manner according to the set dosage to form a mixture. The mixture is then loaded into the sample tube (11) in six layers by a layered compaction method to ensure that the sample density meets the test requirements. S2: Place the sample tube (11) on the permeable stone, reduce the sample air pressure to a vacuum state through the pressurization unit (33), and inject saturated salt water into the sample to make the sample fully saturated. S3: The confining pressure control system (3) and the axial loading system (2) apply graded confining pressure and axial pressure to the sample respectively, and maintain the loading conditions until the sample is consolidated; when the axial and volumetric deformation rates of the sample decrease to the threshold range, the consolidation is considered to be complete. S4: Erosion rate control system (4) introduces distilled water at a set temperature and flow rate into the bottom of the sample to dissolve the salt particles inside the sample and form an erosion liquid; S5: The corrosive liquid is discharged into the storage tank (51). The salt meter (52) monitors the salt content of the discharged corrosive liquid in real time and sends the data to the monitoring system (6). When the salt content is less than 0.1%, it is determined that the salt particles inside the sample have been completely dissolved. The monitoring system (6) continuously records the axial strain, volumetric strain and pore pressure changes of the sample. S6: The sample is subjected to drainage shearing. The axial loading system (2) applies axial force to the sample at a constant displacement rate until the sample reaches the critical state. The monitoring system (6) records the volumetric strain, pore pressure change and stress path of the sample during the entire shearing process.

9. The test method based on salt dissolution erosion according to claim 8, characterized in that, The threshold range mentioned in S3 is less than 0.02% / h.

10. The test method based on salt dissolution erosion according to claim 8, characterized in that, The set temperatures described in S4 are 20℃, 40℃, and 60℃.

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